This invention relates generally to xerographic copying or printing apparatus, and more particularly, it relates to a release agent management system for a heat and pressure fuser for fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a charge-retentive or photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by the application of heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the internally heated fuser roll to thereby effect heating of the toner images within the nip. By closely controlling the heat transferred to the toner, offset of the toner particles from the copy sheet to the fuser roll can be eliminated. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner liquefy and cause a splitting action in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contact surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the apparatus in which the fuser is used.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting," has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the tradename Teflon to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based (polydimethylsiloxane) oils which possesses a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner before it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of abhesive material to a rigid core or substrate such as the solid Teflon outer surface or covering of the aforementioned arrangement.
In attempts to improve at least the perceived quality of the image fused or fixed by a heated roll fuser, such rolls have been provided with conformable surfaces comprising silicone rubber or Viton (trademark of E. I. DuPont of a series of fluoroelastomers based on the copolymer of vinylidene fluoride and hexafluoropropylene). As in the case of the Teflon coated fuser, oil release fluids such as silicone based oils have been applied to the surface of the silicone rubber or Viton to both minimize offsetting and to facilitate stripping. See, for example, U.S. Pat. No. 3,964,431. When the fuser system is one which provides for applying silicone oil to silicone rubber or Viton a relatively low viscosity silicone oil (i.e. on the order of 100-1000 cs) has most commonly been employed. The foregoing type of oil is sometimes referred to as non-functional oil as opposed to function silicone oil, the latter of which interacts with suitable metal particles contained within the rubber or Viton.
Most often, the oil is conveyed to a sump containing an applicator pad and/or roller which applies the oil to the surface of the heated fuser roll.
One of the most common prior art release agent management (RAM) systems uses a wick, one end of which contacts the fuser roll member and the other of which contacts the release agent material to be applied to the fuser member. RAM systems of the foregoing type are known to apply too much release agent or oil to the fuser roll surface, particularly in areas outside the paper path. Excess oil outside the paper path is particularly problematic in machines where various size copies are handled. Accordingly, structures such as a blade as illustrated in U.S. Pat. No. 4,087,676 issued May 2, 1978 to Yasuji Fukase is employed to control the amount of oil ultimately applied to the fuser roll to prevent offset. However, even with such a blade structure excess oil outside the paper path is still a problem and such systems are complex and costly. As described in U.S. Pat. No. 3,883,291 issued May 13, 1975 to Cloutier et al, the oil applied to a fuser roll surface via a wick which contacts the fuser roll surface and the oil supply is controlled by an O-Ring wiper. The Cloutier device does not obviate the shortcomings outlined above.
Another RAM system wherein one end of a wick is immersed in a release fluid and the other end is in contact with the fuser roll is disclosed in U.S. Pat. No. 4,280,443 issued July 28, 1981 to Gregory V. Bogoshian. Bogoshian discloses one solution to the problem of excess oil being applied to the fuser roll. His solution is to move the wick out of contact with the fuser roll during standby periods. It is desirable to be able to control the application of release agent material with a RAM system that continuously contacts the fuser roll. While the device described in Bogoshian solves the problem of excessive oil application it is possible with such a system to apply too little oil.
Another type of prior art RAM system such as that disclosed in U.S. Pat. No. 3,884,181, comprises a supply of oil from which the oil is removed by an applicator roll and conveyed to a wick which is in contact with the fuser roll surface to which it is to be applied. A baffle member is employed to insure a supply of oil on the applicator roll. Such systems are not only complex and expensive by they do not solve the problem of excessive oil being present outside the paper path of one size document nor do they solve the spillage problem inherent in such devices.
The problem of spillage has been solved by structure such as that disclosed in U.S. Pat. No. 3,941,558 issued to Koichi Takiguchi wherein an oil impregnated web is provided for supplying the oil to the fuser roll surface. However, the cost and complexity factors are not obviated by such a structure because of the necessity of motion imparting structure for causing different sections of the web to contact the roll surface. Moreover, the quantity of oil supplied to the roll surface can not be controlled to the degree required by the application contemplated herein.
It will be appreciated that it is most desirable to supply release agent material at a rate commensurate with the machine speed in which the presently contemplated RAM system is to employed. It is further desirable to effect the desired application rate by means of a system which is simple in construction, is relatively inexpensive and which obviates the spilling problem of prior art devices. Heretofore, RAM systems of the prior art have been developed for use in machines having relatively high (i.e. above 2 inches per second) process speeds and are, therefore, inadequate to meet the requirements of the much lower rate requirements of a machine of the type in which applicant's RAM system is to be employed.